Effect of TiO2 particle size on the performance of viologen-anchored TiO2 electrochromic device

The effect of TiO2 particle size on the performance of the electrochromic device (ECD) has been investigated in this work by applying the TiO2 nanoparticles in 7, 15, and 30 nm sizes. The phosphonated viologen, bis(2-phosphonoethyl)-4,4'-bipyridinium dibromide, was anchored on the TiO2 surfaces for the construction of ECD. The ECD derived from 7 nm-sized TiO2 demonstrated the highest contrast ratio with high optical transparency, whereas it showed the slowest switching response. The enhancement of coloration efficiency with decrease of TiO2 particle size is due to the increased amount of the anchored viologen on TiO2 electrode. On the other hand, the relatively slower switching response would be caused by the difficulty of diffusion for the electrolytes and counter-ions through the small pores of the nanocrystalline TiO2 electrode derived from 7 nm-sized nanoparticles.

Influence of the Hofmeister series of anions on the molecular organization of positively ionized monolayers of a viologen derivative

The effects of the Hofmeister series of ions are ubiquitous in chemistry and biology. In this paper specific ion effects on the surface behavior of a viologen dication, namely 1,1(')-dioctadecyl-4,4(')-bipyridilium, are shown. Surface pressure and surface potential vs area isotherms were obtained on aqueous subphases containing potassium salts with several representative counterions in the Hofmeister series (C6H5O3-7, SO2 -4, HPO2-4, Cl-, Br-, NO-3, I-, and ClO-4). The parameters obtained from the compression isotherms (area per molecule, phase transitions, Young modulus, initial surface potential, and variation of the surface potential upon compression) are dependent on the nature of the counterion, indicating ion specificity. Aqueous subphases containing C6H5O3-7, SO2-4, and HPO2-4 anions yield more expanded viologen monolayers and these anions do not effectively penetrate into the monolayer. Brewster angle microscopy was used to map the different phases of the viologen monolayers at the air-water interface. The Langmuir films were also characterized by UV-vis spectroscopy, with quantitative analysis of the reflection spectra supporting an organizational model in which the viologen chromophore undergoes a gradual transition to a more vertical position with respect to the water surface upon compression. A comparison of the tilt angles of the viologen on the different subphases indicates that anions that can more easily penetrate in the monolayer permit the viologen moieties to adopt a slightly more vertical position with respect to the water surface.

Encapsulation of Electron Donor−Acceptor Dyads in β-Cyclodextrin Cavity: Unusual Planarization and Enhancement in Rate of Electron-Transfer Reaction

Interaction of beta-cyclodextrin (beta-CD) with a few novel electron donor acceptor dyads 1a-c and 2a-c, having aryl and flexible methylene spacer groups, has been investigated through photophysical, chiroptical, electrochemical, NMR, and microscopic techniques. Dyads 1a and 1c, with p-tolyl and biphenyl spacer groups, respectively, exhibited significantly decreased fluorescence quantum yields and lifetimes in the presence of beta-CD, while negligible changes were observed for dyad 1b with an o-tolyl spacer. In contrast, spacer-length-dependent significant enhancement in fluorescence quantum yields and lifetimes was observed for dyads 2a-c, with flexible polymethylene (n = 1, 3, 11) spacer groups. Association constants of beta-CD encapsulated complexes have been determined and the contrast behavior observed in these systems is explained through an electron transfer (kET) mechanism based on calculated favorable change in free energy (DeltaGET = -1.27 eV) and the redox species characterized through laser flash photolysis studies. Rates of kET have been estimated and are found to increase ca. 2-fold in the case of dyads 1a and 1c when encapsulated in beta-CD, while significantly decreased kET values were observed for the dyads 2a-c with flexible spacer (ca. 9-fold for 2c). As characterized through cyclic voltammetry, 2D NMR [correlated (COSY) and nuclear Overhauser enhancement (NOESY) spectroscopy], and laser flash photolysis studies, the beta-CD encapsulation of dyads with aliphatic spacer groups leads to the conformational unfolding of a sandwich type of structure, whereas dyads with rigid aryl spacer groups undergo unusual planarization as compared to the uncomplexed dyads, resulting in enhanced electron-transfer reaction between the donor and acceptor moieties.

Electrochemical Detection of DNA Hybridization via Bis-Intercalation of a Naphthylimide-Functionalized Viologen Dimer

The synthesis and DNA binding properties of a bis-naphthyl imide tetracationic diviologen compound NI(CH2)3V(2+)(CH2)6V(2+)(CH2)3NI (where V(2+) = 4,4'-bipyridinium and NI = naphthyl imide, NIV) are described. Binding to thiolated ssDNA and dsDNA immobilized at Au electrodes was characterized using the electrochemical response for reduction of the V(2+) state to the V+ (viologen radical cation) state. Isotherms and binding constants for this molecule to both forms of immobilized DNA were generated in this fashion. The character of the binding isotherm for dsDNA suggests bis-intercalation. Under high saline conditions, the diviologen molecule dissociated 160 times slower from dsDNA compared to ssDNA. Slow dissociation kinetics from dsDNA (kd =7.0 x 10-5 s(-1)) allow this molecule to be used as an effective DNA hybridization indicator.

Molecular complexes based on tetrathiafulvalene and dialkylviologens

Three kinds of molecular complexes based on tetrathiafulvalene (TTF) and dialkylviologens were prepared and their crystal structures elucidated. While TTF-dimethylviologen complex forms a mixed stack arrangement of donors and acceptors in its crystal structure, TTF donors aggregate with long alkyl groups by CH/pi and/or van der Waals interactions in a couple of TTF-heptylviologen complexes.

Quantification of amplified quenching for conjugated polymer microsphere systems

A series of nonaggregating carboxylate-functionalized poly(phenylene ethynylene)s (PPEs) have been synthesized for immobilization via electrostatic adsorption onto Eu3+-polystyrene microspheres with a mean diameter of 0.2 microm. This system is shown to constitute a ratiometric system that measures fluorescence quenching with high fidelity. The fluorescence quenching properties of the polymer-coated particles in response to methyl viologen and a naphthyl-functionalized viologen have been investigated in aqueous solutions to study the influence of electrostatic and hydrophobic interactions with pentiptycene-incorporated as well as macrocycle-containing polymers.

Novel Bifunctional Viologen-Linked Pyrene Conjugates: Synthesis and Study of Their Interactions with Nucleosides and DNA

With the objective of developing efficient DNA oxidizing agents, a new series of viologen-linked pyrene conjugates with the general formula PYLnV(2+), having a different number of methylene spacer units (Ln) was synthesized, and their interactions with nucleosides and DNA have been investigated through photophysical and biophysical techniques. The viologen-linked pyrene derivatives PYL1V(2+) (n =equals; 1), PYL7V(2+) (n = 7), and PYL12V(2+) (n = 12) exhibited characteristic fluorescence emission of the pyrene chromophore centered around 380 nm but with significantly reduced yields when compared to those of the model compound PYL1Et(3)(+). The fluorescence quenching observed in these systems is explained through an electron-transfer mechanism based on a calculated favorable change in free energy (DeltaG(ET) = -1.59 eV), and the redox species characterized through laser flash photolysis studies. Intramolecular electron-transfer rate constants (k(ET)) were calculated from the observed fluorescence yields, and the singlet lifetimes of the model compound and are found to decrease with increasing spacer length. The DNA binding studies of these systems through photophysical, chiroptical, and viscometric techniques demonstrated that these systems effectively undergo DNA intercalation with association constants (KDNA) in the range of 1.1-2.6 x 10(4) M(-1) and exhibit 2:1 sequence selectivity for poly(dG) x poly(dC) over poly(dA) x poly(dT). Photoactivation of these systems initiates electron transfer from the singlet excited state of the pyrene chromophore to the viologen moiety followed by an electron transfer from DNA to the oxidized pyrene. This results in the formation of stable charge-separated species such as radical cations of both DNA and reduced viologen as characterized by laser flash photolysis studies and subsequently the oxidized DNA modifications. These novel systems are soluble in buffer media, stable under irradiation conditions, and oxidize DNA efficiently and selectively through a cosensitization mechanism and hence can be useful as photoactivated DNA cleaving agents.

Charge transfer property of self-assembled viologen derivative by electrochemical quartz crystal microbalance response

Viologen modified electrodes have been extensively investigated with quartz crystal microbalance (QCM), which has been known as a nano-gram order mass detector, because of their highly reversible electrochemical properties, especially the first reduction-oxidation cycle of V2+ <--> V*+. The purpose of this work was to study the charge transfer characteristics of self-assembled monolayer (SAM) by changing electrolyte solutions where the cations and anions are different. The redox peak currents were nearly equal charges during redox processes and showed an excellent linear interrelation between the scan rates and second redox peak currents. The charge transfer of self-assembled viologen monolayer was determined by the mass change during the cyclic voltammetry (CV). The total frequency change was about 17.8 Hz, 19.6 Hz, 9.5 Hz, and 8.4 Hz. From this data, we could know the transferred mass was about 19.0 ng, 20.9 ng, 10.2 ng, and 9.0 ng. Finally, the electrochemical quartz crystal microbalance (EQCM) has been employed to monitor the electrochemically induced adsorption of self-assembled monolayer.

Continuous glucose detection using boronic acid-substituted viologens in fluorescent hydrogels: linker effects and extension to fiber optics

A fluorescent anionic dye and a viologen appended with boronic acids, which serve as glucose receptors, have been synthesized and immobilized into a poly(2-hydroxyethyl methacrylate) hydrogel for use as a continuous glucose monitor. The fluorescence of the dye is modulated by the quenching efficiency of the viologen-based receptor, which in turn is dependent on the glucose concentration. Two monomeric versions of the quencher/receptor unit were prepared and their performance within the hydrogel evaluated. By tethering the quencher/receptor to the hydrogel matrix using a single-point attachment, slightly improved glucose sensing was observed. The hydrogels were tested for their ability to continuously and reversibly detect glucose over the course of several hours. The tests were carried out using a cuvette-based system, as well as a fiber-optic-based configuration. Under physiological conditions (0.1 M phosphate buffer, pH 7.4, 37 degrees C), the fluorescent hydrogels display an excellent dynamic response to glucose concentrations within the biologically significant range (2.5-20 mM).

Radical cations from dipyridinium derivatives: a combined EPR and DFT study

The monoelectronic reduction of 1,1'-dimethyl-2,2'-dicyano-4,4'-bipyridinium (DCMV++) bis-methylsulphate, conducted directly in the cavity of the electron paramagnetic resonance (EPR) spectrometer at room temperature and in DMSO solution, gave the signal of the corresponding radical cation (DCMV.+) whose interpretation has been carried out with the aid of density functional theory (DFT) calculations run at different levels. The model chemistries considered yielded in general hyperfine coupling constants (hfcc) in good agreement with the experimental ones, except for the methyl protons directly bonded to the pyridinium nitrogens. The use of various computational methods accounting for solvent-solute interactions did not give significant improvements with respect to the gas phase results, while the geometry optimizations performed showed that the two pyridinium rings are coplanar in the radical cation but staggered in the parent dication, although the corresponding energy barrier involved is very low.

Ionic Liquids as a Novel Medium for Photochemical Reactions. Ru(bpy)32+/ Viologen in Imidazolium Ionic Liquid as a Photocatalytic System Mimicking the Oxido-Reductase Enzyme†

Ionic liquids are suitable media which stabilize charged intermediates favoring those mechanisms that occur through charge separation. We have used ionic liquids to develop a photocatalytic system to perform the reduction of a carbonyl group to alcohol, thus mimicking the behavior of the reductase enzymes. The photochemical cycle is based on the well-known electron transfer from the Ru(bpy)(3)2+ complex in its excited state, acting as electron donor to MV2+, which acts as electron acceptor. The initial electron transfer process can be promoted upon selective Ru(bpy)(3)2+ excitation by visible light. By means of laser flash photolysis we have provided evidence of the nature and lifetimes of the intermediates involved in the photocatalytic system. Thus, the initial electron transfer between Ru(bpy)(3)2+ triplets and viologen MV2+ forms the MV*+ radical cation, which upon accepting an H* atom from a suitable hydrogen atom donor, forms the corresponding dihydropyridine MVH+ reducing agent.

Competitive photoinduced electron transfer by the complex formation of porphyrin with cyclodextrin bearing viologen

Photoinduced electron transfer between a porphyrin and a new guest cyclodextrin bearing viologen occurs by a supramolecular formation with conformational change of a guest molecule.

Monitoring cyclodextrin-polyviologen pseudopolyrotaxanes with the Bradford assay

Self-assembled multivalent pseudopolyrotaxanes, composed of lactoside-bearing cyclodextrin (CD) rings threaded on linear polyviologen polymers, have been introduced recently as flexible and dynamic neoglycoconjugates. In the course of this research, it was found that polyviologens are responsive to the Bradford assay, which is traditionally highly selective for proteins. The response of the pseudopolyrotaxanes to the Bradford assay was dependant on, and thus indicative of, the degree of threading of the CD rings onto the polyelectrolyte. The assay was then used to report on the threading and dethreading of native and lactoside-bearing alpha-CD rings onto and off of polyviologen chains, a phenomenon which demonstrates the utility of biochemical assays to address problems unique to supramolecular chemistry.

Influence of the Environment on Photoinduced Electron Transfer: Comparison between Organized Monolayers at the Air−Water Interface and Monolayer Assemblies on Glass

Photoinduced electron transfer (PET) has been investigated in organized monolayers at the air-water interface and in monolayer assemblies on glass in an effort to evaluate the influence of solvent reorganization and molecular dynamics on PET. The donor monolayer contained an amphiphilic thiacyanine dye, and the electron acceptors were methyl viologen and dioctadecyl viologen, respectively. The distance dependence is described here by a hard disk model, where an acceptor molecule within a disk with a radius rDA around the excited donor molecule quenches the donor fluorescence due to electron transfer. Acceptor molecules outside the disk are considered ineffective. The critical radius rDA is larger in monolayer assemblies on glass (rDA = 1.97 nm) than at the air-water interface (rDA = 1.15 nm) as evaluated from steady-state fluorescence quenching. This large difference indicates that the time between thermal collisions generating and destroying the energetic match required for electron tunneling from the excited donor molecule to the acceptor is quite different in the two systems that are compared.

The interaction of boronic acid-substituted viologens with pyranine: the effects of quencher charge on fluorescence quenching and glucose response

The fluorescence sensing of several monosaccharides using boronic acid-substituted viologen quenchers in combination with the fluorescent dye pyranine (HPTS) is reported. In this two-component sensing system, fluorescence quenching by the viologen is modulated by monosaccharides to provide a fluorescence signal. A series of viologen quenchers with different charges were prepared and tested for their ability both to quench the fluorescence of HPTS and to sense changes in glucose concentration in aqueous solution at pH 7.4. Both quenching efficiency and sugar sensing were found to be strongly dependent upon viologen charge. The molar ratio between HPTS and each of the viologen quenchers was varied in order to obtain an optimal ratio that provided a fairly linear fluorescence signal across a physiological glucose concentration range. Both the quenching and sugar sensing results are explained by electrostatic interaction between dye and quencher.

Isolation and Characterization of Phenyl Viologen as a Radical Cation and Neutral Molecule

The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV(2+)2Cl(-.)2H2O and PV(.+)PF(6)(-).pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV(.+) to a twist of 36 degrees between the two central rings in PV(2+). The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak pi-dimerization in the solid state and reversible pi-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.

Evaluation of pyranine derivatives in boronic acid based saccharide sensing: significance of charge interaction between dye and quencher in solution and hydrogel

In an ongoing program to synthesize a glucose sensing polymer that could be used for real time glucose monitoring in vivo, we have been exploring the use of boronic acid functional viologens as glucose responsive quenchers for fluorescent dyes. The present study focuses on the effect of ionic interactions between pyranine or its various sulfonamide derivatives and the viologen quenchers. Dyes bearing anionic groups were quenched more efficiently when compared to dyes with nonionic substituents. The anionic dyes in conjunction with the cationic quenchers exhibited a broader range of glucose response both in solution and when immobilized in a hydrogel. The interaction of glucose with the sensing components was similar whether they are soluble or immobilized.

Antibacterial activity of polymeric substrate with surface grafted viologen moieties

An asymmetric viologen, N-hexyl-N'-(4-vinylbenzyl)-4,4'-bipyridinium bromide chloride (HVV), was synthesized and graft copolymerized with commercial PET films. The surface graft concentration of HVV on the PET film is easily controlled by varying the monomer concentration used in the UV-induced graft copolymerization process. The HVV surface functionalized PET film functions as a smart window whose transmittance is reduced upon exposure to light. Concomitantly, the film possesses antibacterial activity, as shown by its bactericidal effect on Escherichia coli (E. coli). The antibacterial activity depends on the concentration of pyridinium groups on the surface and a surface concentration of 25 nmol/cm2 on PET has been shown to be highly effective in killing the bacteria.

A new Co(ii)-metalloviologen-based electrochromic material integrated in thin multilayer films

A metallosupramolecular coordination polyelectrolyte prepared by the reaction of cobalt(II) with a novel bisterpyridine ligand has been assembled as the active component in electrochromic films by sequential deposition using electrostatic layer-by-layer self-assembly.

Optical glucose detection across the visible spectrum using anionic fluorescent dyes and a viologen quencher in a two-component saccharide sensing system

A very general system is described in which anionic fluorescent dyes possessing a wide range of absorbance and emission wavelengths are used in combination with a boronic acid-modified viologen quencher to sense glucose at pH 7.4 in buffered aqueous solution. The present study demonstrates this capability with the use of eleven anionic fluorescent dyes of various structural types. Signal modulation occurs as the monosaccharide binds to the viologen quencher and alters its efficiency in quenching the fluorescence of the anionic dyes. The degree of quenching and the magnitude of the glucose signal were found to correlate roughly with the number of anionic groups on the dye. Optimal quencher : dye ratios were determined for each dye to provide a fairly linear signal in response to changes in glucose concentration across the physiological range.

Metal ion reduction and resultant deposition on viologen-functionalized LDPE films and viologen-containing microporous membranes

Photo-induced reduction of gold and platinum metal salt solutions was carried out using viologen graft copolymerized on low-density polyethylene (LDPE) films and viologen-containing poly(vinylidene fluoride) (PVDF-PVBV) microporous membranes. The effects of the UV irradiation time and concentration of the metal salt solutions on the metal ion reduction process and the resultant metal deposition on the polymeric substrates were investigated. The metal-polymer composites were characterized using X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy (SEM and TEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-visible absorption spectroscopy. The amount of metal uptake, the state of the metal, and the size of the metal particles were found to be strongly dependent on the UV irradiation time and the type and concentration of the metal salt solution. The microporous structure and the high viologen content of the PVDF-PVBV membrane constitute an effective matrix for metal ion reduction and preparation of metal nanoparticles.